The present invention relates to an improved capacitive pressure sensor having a diaphragm structure for capacitively sensing changes in pressure under measurement.
Conventionally, a capacitive pressure sensor of a type shown in a cross-sectional view of FIG. 1 has been proposed. Specifically, a pressure sensor 36 includes a cover glass 32 typically made of heat resistant glass, such as that sold under the trademark Pyrex glass or the like having a cavity in which a fixed electrode 31 is deposited on the bottom thereof, and a silicon substrate 34 on which a movable electrode 33 is deposited at a position opposite to the fixed electrode 31. The cover glass 32 and the silicon substrate 34 are bonded with each other by anode-bonding their peripheral portions or bonding surfaces 35 such that the electrodes face in parallel with each other, thus forming a diaphragm sensing portion D. This pressure sensor is used with the silicon substrate 34 or the movable electrode 33 being placed to receive pressure under measurement (P1) and with the fixed electrode 31 being placed to receive the atmospheric pressure (P2).
In operation, an increase in pressure under measurement P1 causes the diaphragm sensing portion D of the capacitive pressure sensor 36 to be deformed, which results in changing the gap G between the fixed electrode 31 and the movable electrode 33 (the gap is narrowed in this case). Then, a varying capacitance value of a capacitor formed of the fixed electrode 31 and the movable electrode 33, caused by the changed gap G, is sensed to detect the pressure under measurement P1.
The conventional capacitive pressure sensor thus constructed, however, has a problem that changes in environmental factors such as humidity and so on cause corresponding fluctuations in the dielectric constant of air existing in the gap G between the fixed electrode 31 and the movable electrode 33. This leads to the capacitance value of the capacitor formed of the fixed electrode 31 and the movable electrode 33 varying due to factors other than changes in pressure under measurement.
Laid-open Japanese Patent Application No. 63-308529 (1988) proposes, as a solution to the problem mentioned above, a capacitive pressure sensor having a reference capacitor for removing error factors due to environmental changes such as changes in humidity.
This capacitive pressure sensor has a sensing capacitor, the capacitance value of which changes in response to pressure under measurement, and a reference capacitor, the capacitance value of which does not change even if pressure under measurement varies. Since the same atmospheric pressure is introduced into gaps between electrodes of the respective capacitors, error factors possibly caused by environmental changes can be removed by subtracting the capacitance value of the reference capacitor from the capacitance value of the sensing capacitor.
Although the capacitive pressure sensor constructed as proposed in Laid-open Japanese Patent Application No. 63-308529 is effective in providing accurate measurements to some degree, it cannot deal with errors caused by dew condensation on the surface of electrodes constituting the capacitors and small dust particles introduced into the gap between the electrodes. The dew condensation is regarded as an inevitable phenomenon when a temperature of a gas or liquid to be measured is lower than the atmospheric temperature. Also, the introduction of dust particles is an inevitable problem as long as the pressure sensor has a atmospheric pressure introducing hole.
In addition to the above-mentioned environmental changes, the foregoing type of pressure sensors possibly suffer from errors due to aging change of residual stress on the bonding interface between different materials used for the sensor structure, e.g., the cover glass and the silicon substrate in the prior art example.
These problems give rise to erroneous measurements of pressure, so that accurate and reliable pressure measurement is rendered impossible.